1. Field of the Invention
The present invention pertains to a medical ultrasonic diagnostic apparatus and an ultrasonic imaging method. More particularly, the present invention relates to an ultrasonic diagnostic apparatus and an ultrasonic imaging method utilizing non-linearity when ultrasonic signals propagate tissues of a living body or non-linearity of a contrast medium when the contrast medium is administered to the living body, carrying out diagnosis, thereby sampling a non-linearity component of a transmission ultrasonic wave from a receiving signal and providing useful diagnostic information concerning tissue information or blood flow information.
2. Description of Related Art
In the field of the medical care, an ultrasonic diagnostic apparatus comes to be one of essential imaging modalities. Because the ultrasonic diagnostic apparatus has its superiority in that the apparatus is comparatively small in size and inexpensive, free of X-ray exposure, and enables blood flow imaging using an ultrasonic Doppler technique.
In particular, in recent years, attention has been paid to a tissue harmonic imaging technique for sampling and imaging a harmonic component (such as a second harmonic component, for example) that is a non-fundamental component of a transmission ultrasonic wave produced by non-linear propagation of tissues relevant to an ultrasonic wave. This technique is advantageous in that a harmonic component is extracted, and transmission beams can be emitted more finely, a side lobe can be reduced, and further, a high quality image can be produced.
In addition, there is known a harmonic imaging technique utilizing non-linearity of an ultrasonic contrast medium concerning ultrasonic reflection, thereby acquiring dynamic information on blood flow. The non-linearity of a contrast medium is stronger than that of tissues of a living body. Thus, a harmonic component of a transmission ultrasonic wave is sampled from a receiving echo, and is imaged, thereby making it possible to generate an image with a better contrast between a tissue and the contrast medium.
In the harmonic imaging, a filter is used to sample a harmonic component. However, in actuality, with respect to the fundamental component and harmonic component of a transmission ultrasonic wave contained in a receiving echo, substantial parts of frequency ranges are superimposed on each other on a frequency axis as shown in FIG. 12. Thus, there is a problem that the fundamental component and harmonic component cannot be separated expectedly even using the filter. Namely, if an attempt is made to sample only a harmonic component, a cutoff frequency of the filter must be set to fhigh. By setting this, the harmonic component is narrow in bandwidth, the resolution in an image depth direction is degraded, and signal intensity is lowered as well. In contrast, when such cutoff frequency is set to flow in the figure, a harmonic component in a wide bandwidth can be sampled, however, a large amount of fundamental components coexist. Thus, an increase amount of artifacts such as side lobe of the fundamental appear on an image.
In such a dilemma, in recent years, there has been known an imaging technique called a pulse inversion technique in the publication “Non-linear Propagation of Ultrasonic Pulses, Tomo-o Kamakura, et al., (Report of The Institute of Electronics & Communication Engineers US 89-23, p. 53). This pulse inversion technique is a technique that: individually transmits two ultrasonic waves, which their polarities is inverted to each other, for every scanning line for forming one image; and adds RF or IQ data, which is derived from transmission of one of the two ultrasonic waves having a non-inverted polarity, to RF or IQ data, which is derived from transmission of the other of the two ultrasonic waves having an inverted polarity, thereby producing image data for one frame.
According to this pulse inversion technique, in the case of a static scatterer and reflecting echo source, only a harmonic component can be sampled from frequency ranges superimposed on a fundamental component by the addition. That is, a harmonic component in a wide bandwidth can be sampled.
However, in the case of that a living body is a diagnostic subject, it is impossible to avoid movements of viscera. In the case of using the pulse inversion technique, it is impossible to exhibit the function of the elimination of the fundamental component by the addition. The fundamental component remains. And this remainder component appears on an image as a motion artifact.
According to this pulse inversion technique, although it is required to generate an ultrasonic pulse having its positive and negative polarities completely inserted from its principle of operation, such pulse generation is technically difficult in actuality. Therefore, even if the described motion of a living body is almost ignored, the remaining component of the fundamental component caused by addition occurs, and thus, the fundamental component cannot be completely or fully separated from the harmonic component.